Imidacloprid (IMI), currently the most important insecticide acting at the nicotinic acetylcholine receptor (nAChR), is the first of a large family of neonicotinoids to be commercialized in the next few years with concomitant human exposure. 1-(6-chloro-3-pyridyl)methyl-2-nitromethylene-imidazolidine (I) is a very potent insecticide and is 6-fold more effective than (IMI) in displacing [3H]IMI from its binding site in the housefly acetylcholine receptor, and is 4-fold more potent in killing houseflies. 1-(6-chloro-3-pyridyl)methyl-2-iminoimidazolidine (II) is a metabolite of (IMI) in rats, plants, and soil. While it lacks the insecticidal potency of IMI, both compounds (I) and (II) have a high toxicity to mammals and have a very high affinity to the nAChR in rat brains. This study is to label (I) and (II) at high specific activity to investigate the basis of the outstanding effectiveness of compound (I) and to examine in depth the toxicity of both compounds.